Carbonaceous sorbent and process for the production thereof

ABSTRACT

The invention concerns a carbonaceous sorbent in powder or grain form for the dry cleaning of waste gases from thermal processes. The sorbent includes carbon adsorbents from the group of activated carbon and/or brown coal cokes which are modified with sulfur and/or sulfur compounds. The sorbent is distinguished in that the specific surface area of the carbon adsorbents in m 2 /g in relation to the pore volume of the micropores in cm 3 /g is between 2400 and 2700.

Carbonaceous sorbent and process for the production thereof Theinvention concerns a carbonaceous sorbent in powder or grain form forcleaning waste gases from thermal processes and a process for theproduction of a carbonaceous sorbent for dry waste gas cleaning.

The invention concerns in particular a carbonaceous sorbent in powder orgrain form for the dry cleaning of waste gases from thermal processes,including carbon adsorbents from the group of activated carbon and/orbrown coal cokes which are modified with sulfur and/or sulfur compounds.

The invention further concerns a process for the production of acarbonaceous sorbent for dry waste gas cleaning including theimpregnation of carbons from the group of activated carbon and/or browncoals with an aqueous sulfur-bearing solution.

Carbon adsorbents are used in particular for the cleaning of waste gasesfrom metallurgical/secondary-metallurgical processes. Waste gases fromthose processes contain polyhalogenated dibenzodioxins and dibenzofuransas well as heavy metals, in particular mercury. Those waste gases alsoentrain process dusts which can catalyse carbon oxidation and cantrigger off smouldering fires in the filter installations.

A process for the cleaning of waste gases from sintering installations,smelting works or secondary-metallurgical smelting plants as well as asorbent in dust form for the dry cleaning of such waste gases is knownfor example from DE 199 40 683 B4. In that process sorbents in powderform are brought into contact with the waste gas flow withoutpreliminary removal of dust from the waste gas flow, using coke frombrown coal as the sorbent. That process is carried out in the form of aper se known flying stream adsorption procedure. In that case theadsorbent in powder form is metered directly into the flow of waste gasand then separated off together with the process dust in filterinstallations.

In particular the brown coal coke dust used in the process in accordancewith DE 199 40 683 enjoys a particularly high level of effectiveness interms of separation of pollutants and contaminants, inter alia becauseof the excellent adsorption properties of brown coal coke.

It will be noted however that the excellent adsorption performance ofbrown coal coke dust entails an increased tendency to catching fire.Mixed with the catalytically active process dusts which occur in thewaste gas, there can be an increased tendency for the dust separated offin the filter installation to catch fire so that the use of brown coalcoke dust and equally the use of powder activated carbons is onlylimitedly possible.

Therefore DE 199 40 683 describes a process for inerting brown coal cokedust, which in principle has proved its worth. Zeolites orzeolite-bearing mineral rock are added to the brown coal coke dust,wherein the brown coal coke and the zeolite are so adjusted independence on the type, composition and amount of the process dustcontained in the waste gas in such a way as to ensure that the solidsare rendered inert.

It will be noted however that a disadvantage of the known process isthat, when dealing with highly reactive process dusts as occur forexample in scrap and copper preparation, considerable amounts of zeolitematerial have to be added to afford an adequate inerting effect, wherebythe adsorption capability of the mixture is greatly reduced as, byvirtue of their polar properties, zeolites do not or only inadequatelyadsorb elementary mercury as well as dioxins/furans.

EP 1 025 894 describes a process in which brown coal coke in powder formis doped with catalysis inhibitors which are intended to preventmetal-catalysed carbon oxidation emanating from the process dust. Thatprocess is disadvantageous insofar as the proposed substances do notexhibit inhibition of the tendency to ignition, which is equallyeffective for all metal catalysts, and they do not improve the sorptionof elementary mercury on the brown coal coke.

A sorbent and a process of the kind set forth in the opening part ofthis specification are known for example from DE 199 36 930 A1. DE 19936 930 A1 discloses a process and an apparatus for the separation ofmercury from hot pyrolysis gases, wherein mercury-binding substances areintroduced into the pyrolysis gas, thus affording solid mercurycompounds. They are separated off at fine dust filters. Themercury-binding substances include sulfur, sulfur-doped activatedcarbons, hearth furnace cokes, bentonites, zeolites, trasses and/orbrick powder.

A variant of the process proposed in DE 199 36 930 provides thatsulfuric acid, hydrogen fluoride and/or hydrogen chloride are introducedas a gas or a liquid into the pyrolysis gas, mixed with the pyrolysisgas and caused to react with the metallic mercury. The resultingreaction products are separated off at hot gas filters.

In another variant of the process as described in DE 199 36 930sulfur-doped activated carbons or hearth furnace cokes, zeolites,bentonites, trasses or other organic fine dusts having a sulfur contentof between 10 and 30% by weight are introduced into the pyrolysis gas.It is further proposed therein that sulfur and activated carbon shouldbe introduced separately from each other or in the form of a mixtureinto the pyrolysis gas. Instead of adding sulfur it is alternativelyproposed that sulfur-delivering substances can be used, for examplesodium tetrasulfide and sodium thiosulfate. The decomposition of thosesubstances provides that elementary sulfur is separated off in finelydivided form and thus facilitates the reaction with mercury. Thesubstances thus introduced into the pyrolysis gas are to be removed fromthe gas with suitable filters.

Cokes or activated carbons doped with sulfur or sulfur compounds have animproved separation capability in relation to heavy metals, inparticular they are improved in regard to the separation of elementarymercury. It has been found however that those properties are to thedetriment of adsorption capability for organic compounds, in particularfor dioxins and furans. For that reason the sorbents described in DE 19936 930 are not suitable for exhaust gas cleaning downstream of metallicand/or secondary-metallic installations. In addition carbon adsorbentshave an increased tendency to catch fire, which is undesirable inconnection with carbon oxidation-catalysing constituents in the processdust or in the waste gas.

Known sorbents therefore either have particular properties for theseparation of heavy metals or particular properties for the separationof organic compounds.

Therefore the object of the invention is to provide a carbonaceoussorbent which has a high level of sorption capability for elementarymercury and at the same time a high adsorption capacity for organiccontaminants and pollutants, in particular for polyhalogenateddibenzodioxins and/or dibenzofurans. Furthermore the invention seeks toprovide that the sorbent is safe in terms of its tendency to catch fire.

A further object of the invention is to provide a suitable process forthe production of such a sorbent.

The object of the invention is firstly attained by a carbonaceoussorbent in powder or grain form for the dry cleaning of waste gases fromthermal processes, including carbon adsorbents from the group ofactivated carbon and/or brown coal cokes which are modified with sulfurand/or sulfur compounds, wherein the sorbent according to the inventionis distinguished in that the specific surface area of the carbonadsorbents in m²/g, with respect to the pore volume of the micropores incm³/g (index H) is between 2400 and 2700.

In a preferred variant of the sorbent according to the invention theindex H is between 2500 and 2650.

The index H is the quotient of the specific surface area determined inaccordance with what is referred to as the BET method and the microporevolume in accordance with the IUPAC standard (micropores of a width(diameter) of ≦2 nm in cm³/g). The micropore volume in that diameterrange is determined by gas adsorption in accordance with theDubinin-Radushkevich isomer model. BET measurement involves a widespreadanalysis method of determining the magnitude of surface areas by meansof gas sorption (Brunauer, Emmet and Teller).

Surprisingly it has been found that a carbonaceous sorbent in powderform, the carbon adsorbents of which are modified with sulfur and/orsulfur compounds, and the specific surface area of which in relation tothe pore volume is of the above-mentioned order of magnitude, withapproximately the same adsorption performance for organic substances,has a comparatively lower tendency to ignition than carbon adsorbentswhich are doped with sulfur and sulfur compounds and the index H ofwhich is outside the claimed range. The impregnation process accordingto the invention is distinguished by lesser blocking ofadsorption-effective micropores for dioxins/furans while reactivity withmercury or other metals remains substantially unaffected.

As is also described hereinafter the above-mentioned ratio of thespecific surface area to the pore volume is only achieved by aparticular treatment of the carbon adsorbents. In the case of theconventional spray or dip impregnation of the carbon adsorbents withsulfur-bearing compounds in aqueous solution, the result is a reducedadsorption capability for organic contaminants and pollutants. Inparticular the pore volume is greatly reduced, which directly entails areduction in the adsorption capacity for organic pollutants andcontaminants. By virtue of the greater pore volume, a greater chemicalreactivity would be expected from a carbonaceous sorbent whose index H(specific surface area in relation to the pore volume) assumes theabove-mentioned order of magnitude. Surprisingly however it has beenfound that the carbonaceous sorbent according to the invention isuncritical in regard to its tendency to ignition.

Preferably brown coal cokes are provided as carbon adsorbents.

A further preferred variant of the sorbent according to the inventionprovides that the carbon adsorbents are modified with a polysulfide,preferably with an alkali metal polysulfide.

The process for the production of a carbonaceous sorbent for dry wastegas cleaning according to the invention includes the impregnation ofcarbons from the group of activated carbons and/or brown coals with anaqueous sulfur-bearing solution, wherein the process is distinguished inthat the sulfur-bearing solution is added with agitation of the carbonsin a closed mixing container under an increased pressure or a reducedpressure.

As mentioned hereinbefore the aqueous solution can contain apolysulfide, preferably the aqueous solution can contain a disodiumtetrasulfide.

In a particularly preferred variant of the invention it is provided thatthe treatment is effected in the mixing container under a controlledaddition of oxygen.

The oxygen concentration in the mixing container can be so adjusted thatthe polysulfide experiences in-situ partial oxidation to affordelementary sulfur.

With the process according to the invention it is particularlyadvantageous that the reaction heat which occurs upon agitation of theconstituents of the mixture in the closed container and also thefrictional heat introduced into the material being mixed by theintensive thorough mixing effect cause such a rise in temperature in thematerial being mixed that the solvent partially dries up during themixing operation. That is sufficient to produce a product which ispourable or capable of trickle flow and which does not require anypost-drying operation.

In a particularly preferred feature the sulfur solution is added with aproportion of between 1 and 15% by mass, preferably with a proportion ofbetween 1 and 7% by mass.

The invention is described hereinafter by means of a number of examples.

EXAMPLE 1

500 parts by weight of brown coal coke dust (hearth furnace coke (HFC)Super RWE Power AG, Cologne) are sprayed in an Eirich mixer (Type R02,capacity 5 I, from G Eirich Maschinenfabrik) with 65.8 parts by weightof a 40% Na₂S₄ solution, which by calculation corresponds to an Na₂S₄content of the impregnated coke of 5% by mass. The Na₂S₄ solution ismeteredly fed into the mixer within 68 s in the form of a spray conewhich is produced by a hollow cone nozzle (0.2 mm) under a pressure of14 bars. The mixing pot involves a rotary speed of 90 min⁻¹, the vaneagitator rotates in opposite relationship at 3000 min⁻¹. Initially airat ambient temperature was to be found in the mixer. After opening ofthe mixer a temperature of 80° C. was measured. The water content (DIN51718) of the sample was determined as 4.4% by mass. The coke sample wascapable of trickle flow. The Na₂S₄ content of the sample, calculatedback from the measured sulfur content (DIN 51724) was 5.16% by mass.

As a measurement in respect of the tendency to ignition, taking amixture of 20 parts by weight of the impregnated coke dust and 80 partsby weight of a metallurgical process dust, the combustion number thereofwas determined in accordance with VDI 2263 (see for example Heschel etal: Ein verbessertes Mess- und Auswerteverfahren zur Bestimmung derBrennzahl von Stäuben nach VDI-Richtlinie 2263. Gefahrstoffe—Reinhaltungder Luft 63 (2003), 469-474) [‘An improved measurement and assessmentprocess for determining the combustion number of dusts in accordancewith the VDI Guideline 2263. Danger substances—keeping the air clean’].In that case the prismatically shaped test bulk material of thecoke-process dust mixture is caused to ignite with a glowing platinumcoil and the nature of propagation of the fire is characterised byspecifying a combustion number. With a combustion number of 4 theinitiated smoldering fire is propagated over the entire test material. Acombustion number of 3 is characterised by local burning or smolderingwith an extremely slight degree of propagation and is thereforeclassified as harmless from the point of view of safety technology. Anaverage combustion number of 2 was determined (brief ignition, rapidextinction).

The adsorption capability in respect of the impregnated coke sample for1,3-dichlorobenzene and toluene in a vapor atmosphere saturated at 20°C. was measured (static adsorption). The two organic compounds arerepresentative of the class of substances of polyhalogenateddibenzodioxins/furans. The adsorption capacity for mercury vapor wasmeasured by means of a laboratory fixed bed adsorber. The measurementconditions were as follows: entry concentration for elementary mercury850 μg/m³; adsorber temperature 90° C.; vacuum gas speed 1.7 cm/s;carrier gas with 14% by volume oxygen, balance nitrogen, watervapor-saturated at 8° C.

In addition the coke sample was acted upon with a waste gas from anindustrial installation for recycling steel works dusts, which containeddioxins/furans as well as mercury in predominantly elementary form andwhich was at a temperature of 90° C. The degree of separation fordioxins/furans as well as the loading with mercury of the impregnatedcoke sample and in comparative terms for the untreated coke wasdetermined under identical measurement conditions.

The process according to the invention produced a brown coal coke havingadvantageously modified use properties for waste gas cleaning, inparticular the reactivity thereof in a mixture with metallurgicalprocess dust was reduced from the original combustion number 4 to thecombustion number 2. The Hg loading rose in the laboratory test from 20μg/g to 142 μg/g. Under the waste gas conditions in the industrialinstallation the Hg loading was increased from 20 μg/g for the untreatedcoke to 32 μg/g for the impregnated coke sample, while the degree ofseparation for dioxins/furans was reduced by the impregnation operationfrom 97.3% to only 75.6%, that is to say by 20% relative (Table 1). For1,3-dichlorobenzene and toluene, 1.5 mmol/g and 2.2 mmol/g respectivelywere measured, corresponding to 10% less than for the untreated coke. Itcan be seen from FIG. 1 that the frequency of the micropores which arenot accessible for stearic reasons to the pollutants or contaminants(≦0.5 mm pore width) is significantly less than in the initial coke. Themoisture content of the coke sample, due to the rise in temperature inthe mixer, fell from initially 7.4% by mass to 4.5% by mass so that theresult obtained was a coke capable of trickle flow.

EXAMPLE 2

500 parts by weight of brown coal coke dust were sprayed as described inembodiment 1 with 138.9 parts by weight of a 40% Na₂S₄ solution, whichin terms of calculation corresponds to an Na₂S₄ content of theimpregnated coke of 10% by mass. The Na₂S₄ solution was meteredly addedto the mixer within 144 s. The Na₂S₄ content calculated back from thesulfur content of the coke sample was 9.72% by mass. The temperaturerose to 95° C.

The result obtained was a brown coal coke, the reactivity of which wasreduced in the mixture with metallurgical process dust from a combustionnumber 4 to a combustion number 2. The Hg loading rose in the laboratorytest from 20 μg/g to 139 μg/g. Under the waste gas conditions of theindustrial installation the Hg loading showed practically no increase(Table 1). The equilibrium loading was 1.1 mmol/g (−28%) for1,3-dichlorobenzene and 1.9 mmol/g (−20%) for toluene. The moisturecontent of the coke sample was reduced by the drying effect during themixing operation from initially 14.3 to 5.2% by mass so that a cokecapable of trickle flow was also obtained.

EXAMPLE 3

The coke sample produced in accordance with embodiment 1 was acted uponin a laboratory fixed bed adsorber at 90° C. with a model gas comprising4500 ppmv SO₂, 0.4% by volume H₂O, 14% by volume O₂ and 880 μg Hg/m³ at90° C. In contrast to the measurement conditions in embodiment 1(without SO₂ in the model gas), all mercury was removed from the gas dueto the presence of SO₂ over a period of 12 h.

EXAMPLE 4 Comparative Test

The following example documents the known state of the art in terms offluid impregnation of adsorbents and serves to clearly illustrate themode of operation of the process according to the invention.

20 parts by weight of brown coal coke dust (HFC Super RWE Power AG,Cologne) are added to the aqueous Na₂S₄ solution which is present inexcess and the concentration of which was set to 1.5, 10 and 20%respectively, and intensively mixed therewith. After a stand time forthe suspension of one hour, it was separated by way of a paper filterand the filter residue dried in a drying cabinet at 105° C. for 4 h. Thefollowing were determined in respect of the sample which was ground upin the mortar, as set forth in Example 1: sulfur content, combustionnumber, 1,3-DCB and toluene equilibrium loading respectively, and Hgloading. The results are shown in Table 2. The result obtained was abrown coal coke, the reactivity of which only reaches the combustionnumber of 2 at a sulfur content of 7.43% by mass. The higher Na₂S₄concentration which is to be found in comparison with Example 1 on thecoke (10.09 vs. 5.16% by mass) has the result that more micropores areblocked (FIG. 2) and as a consequence thereof the absorption capacityfor 1,3-DCB and toluene respectively is reduced more greatly as apercentage than in Example 1.

TABLE 1 Results of the application tests with a waste gas from anindustrial installation for recycling steel works dusts AdsorbentAdsorbent Raw gas (doped) (untreated) BCCD1833/1 + 5% by mass Na₂S₄(spray Embodiment 1 impregnated) BCCD1833/1 Mercury Hg loading 400-700μg/m³ 32 20 adsorbent [μg/g] Hg loading increase — +56.1 — by [%]Dioxins/furans PCDD/F   0.037 ng/m³ 0.009 (clean gas) 0.001 (cleanconcentration gas) (ng/m³)¹ PCDD/F degree of — 75.6 97.3 separation [%]BCCD1833/1 + 10% by mass Na₂S₄ (spray Embodiment 2 impregnated)BCCD1833/1 Mercury Hg loading 400-800 μg/m³ 24.5 24.3 adsorbent [μg/g]Hg loading increase +0.8 by [%] ¹Toxicity equivalent 17th BlmSchV I-TEQ(limit value 0.1 ng/m³).

TABLE 2 Comparison of the carbon adsorbents modified in accordance withthe invention of Example 1 with dip impregnation of the carbonadsorbents of Example 4 (state of the art) and with untreated brown coalcoke Na₂S₄ content for combustion number = 2 Toluene [% by DCB loadingloading Hg loading mass] [mmol/g] [mmol/g] [μg/g] Example 1 5.16 1.5 2.2142 Example 4 10.09 1.2 1.8 140 Micropore vol. ≦2. nm Index H BET [m²/g][cm³/g] s [nm] [m²/cm³] Example 1 137.3 0.0523 1.435 2.621 Example 425.3 0.0108 2.334 2.330 BCCD 277.7 0.1079 1.123 2.573 1833/1 (untreated)

As can be seen from Table 2 the comparison of an adsorbent produced inaccordance with Example 1 with a sorbent produced in accordance withExample 4 (state of the art) shows for the sorbent according to theinvention with about half the Na₂S₄ doping (5.16% by mass: 10.09% bymass), even better loadability for organic pollutants and contaminants(DCB loading 1.5: 1.2/toluene loading 2.2: 1.8) with approximately thesame mercury loadability. Admittedly the process in accordance with thestate of the art (Example 4) also achieves a combustion number of 2 butthat is with an incomparably higher level of use of sodium tetrasulfideand with a lower level of loadability for organic contaminants andpollutants, which can also be seen from the adsorption pore widthidentified by ‘s’. The mean adsorption pore width of untreated browncoal coke is 1.123 nm, that of the sorbent in accordance with the stateof the art is 2.334 nm and that of the sorbent according to theinvention is 1.435 nm.

1. A carbonaceous sorbent in powder or grain form for the dry cleaningof waste gases from thermal processes, including carbon adsorbents fromthe group of activated carbon and/or brown coal cokes which are modifiedwith sulfur and/or sulfur compounds, characterised in that the quotient(index H) of specific surface area of the carbon adsorbents in m²/g andthe pore volume of the micropores in cm³/g is between 2400 and
 2700. 2.A sorbent as set forth in claim 1 characterised in that the index H isbetween 2500 and
 2650. 3. A sorbent as set forth in claim 1characterised in that brown coal cokes are provided as carbonadsorbents.
 4. A sorbent as set forth in claim 1 characterised in thatthe carbon adsorbents are modified with a polysulfide, preferably withan alkali metal polysulfide.
 5. A process for the production of acarbonaceous sorbent for dry waste gas cleaning including theimpregnation of carbons from the group of activated carbons and/or browncoals with an aqueous sulfur-bearing solution characterised in that thesulfur-bearing solution is added with agitation of the carbons in aclosed mixing container under an increased pressure or a reducedpressure.
 6. A process as set forth in claim 5 characterised in that theaqueous solution contains a polysulfide.
 7. A process as set forth inclaim 5 characterised in that the aqueous solution contains an alkalimetal polysulfide.
 8. A process as set forth in claim 5 characterised inthat the aqueous solution contains a disodium tetrasulfide.
 9. A processas set forth in claim 5 characterised in that the treatment is effectedin the mixing container under a controlled addition of oxygen.
 10. Aprocess as set forth in claim 6 characterised in that the oxygenconcentration in the mixing container is so adjusted that thepolysulfide experiences in-situ partial oxidation to afford elementarysulfur.
 11. A process as set forth in claim 5 characterised in that thesulfur solution is added with a proportion of between 1 and 15% by mass,preferably with a proportion of between 1 and 7% by mass.